1. Field of the Invention
The invention relates to a drawing-processed high strength and high thermal conductivity copper alloy tube and a method for producing the same.
2. Description of Related Art
Copper having excellent thermal conductivity has been used for tube members (hereinafter, referred to as a pressure-resistance and heat-transfer vessel in the general term) such as a header, a distribution joint, a dryer, a muffler, a filter, and an accumulator used for heat exchangers for such as a refrigerator, a freezer, an air conditioner, and a boiler, since previous times. Generally, a high strength and high thermal conductivity copper alloy tube (hereinafter, referred to as a high function copper tube) made of phosphorus deoxidized copper (JIS C1220) based on pure copper excellent in thermal conductivity, heat resistance, and brazing property have been used. The pressure-resistance and heat-transfer vessels are pressure vessels having a shape in which both ends or one end of the high function copper tube are drawn. An outer diameter of the pressure-resistance and heat-transfer vessels is 1.5 or more times as large as that of the tubes made of phosphorus deoxidized copper and the like connected to the pressure-resistance and heat transfer vessels, and a refrigerant or the like passes through the inside thereof. Accordingly, high internal pressure is applied to the pressure-resistance and heat-transfer vessel. Heat resistance represents that something is hardly recrystallized even if heated at a high temperature, or that crystal grains hardly grow although a few might be recrystallized, thereby keeping high strength. Specifically, copper alloy having high heat resistance is hardly recrystallized and the strength thereof slightly decreases, even when the copper alloy is heated to about 400° C., which is a recrystallization temperature of pure copper, and even when the copper alloy is heated to 600° C. to 700° C. at which crystal grains of pure copper, start coarsening and strength thereof decreases. In addition, when the copper alloy is heated to about 800° C. or higher at which crystal grains of pure copper are significantly coarsened, the copper alloy is recrystallized. However, the crystal grains of the copper alloy are fine, and the copper alloy has high strength.
Processes for producing the high function copper tube are as follows. [1] Cast cylindrical ingot (billet, outer diameter: about 200 mm to about 300 mm) is heated to 770 to 970° C., and then is hot-extruded (outer diameter: 100 mm, thickness: 10 mm). [2] Immediately after the extrusion, the ingot is air-cooled or water-cooled in the temperature range from 850° C. or the temperature of the extrusion tube after the extrusion to 600° C. at an average cooling rate of 10 to 3000° C./second. [3] Afterwards, in regards to a cold state, a tube is produced with an outer diameter of about 12 to 75 mm and a thickness of about 0.3 to 3 mm by tube rolling (processed by a cold reducer, etc.) or drawing (processed by bull block, combining, die drawing, etc.). Mostly, in the course of the process of the tube rolling or the drawing, a heat treatment is not performed. However, there is a case in which annealing is performed thereon at 400 to 750° C. for 0.1 to 10 hours. In addition, there is a method of obtaining an unprocessed tube from a cylindrical continuous cast having an outer diameter of 50 to 200 mm by in a tube rolling method processed in a hot state of about 770° C. or higher or by the Mannesmann method, instead of the hot extruding, using the heat generated by the plastic working process, thereby obtaining a tube member having the size obtained in a cold state as described above. Finally, both ends or one end of the tube member obtained by the tube rolling or the drawing are drawn by a spinning process or the like, thereby producing a pressure-resistance and heat-transfer vessel.
FIG. 1 shows a side section of the pressure-resistance and heat-transfer vessel. In the specification, terms of parts of the pressure-resistance and heat-transfer vessel 1 drawn by the spinning process are defined as follows. An outer diameter of an unprocessed tube that is not spinning-processed is defined as D.
UNPROCESSED TUBE PORTION 2: A part that is not spinning-processed.
DRAWING TUBE PORTION 3: A part that is drawn with a predetermined diameter by a spinning process.
PROCESS CENTER PORTION 4: The drawing tube portion and a part within a half of a distance from the drawing tube portion to an outer periphery of the unprocessed tube portion.
PROCESS END PORTION 5: A part within a distance D/6 from the outer periphery inward in the end surface of the unprocessed tube portion. Thicknesses of the drawing tube portion 3, the process center portion 4, and the process end portion 5 are 2 to 3 times of the thickness of the unprocessed tube at the thickest part by a spinning process. The thickness of the process end portion gets thinner toward the end of the process end portion.
HEAT-INFLUENCED PORTION 6: In the unprocessed tube portion, a part within a distance D/6 from the process end portion toward the unprocessed tube portion, assuming a part where the temperature is increased to 500° C. or higher by process heat. A part where the temperature is not increased to 500° C. or higher is not included in the heat-influenced portion.
STRAIGHT TUBE PORTION 7: A part of the center of the unprocessed tube portion from a part within a distance D/2 from the process end portion toward the unprocessed tube portion, assuming a part where the temperature is not increased to 500° C. or higher by process heat.
DRAWING-PROCESSED PORTION 8: a part including both of the process end portion 5 and the heat-influenced portion 6.
Terms of parts of the pressure-resistance and heat-transfer vessel that is subjected to drawing by “Hera-shibori” (“hera” represents a jig in the shape of rods or plates, or a metallic spatula, which is pressed against the spinning material to shape, and “shibori” means drawing), swaging, or the like are defined in the same manner described above. When heat is not generated by the drawing process, the heat-influenced portion is a part within a distance D/6 from the process end portion toward the unprocessed tube portion. In the specification, a drawing process such as a “Hera-shibori” process, a swaging process, and a roll forming process, in which little heat is generated, is defined as a cold-drawing process.
In a spinning process for producing a pressure-resistance and heat-transfer vessel having a general shape, a material temperature of a processed portion reaches a high temperature of 700 to 950° C. by process heat. The process center portion 4 drawn by the spinning process is heated to 800° C. or higher and thus is recrystallized, thereby decreasing strength. Since the thickness of the process center portion 4 becomes large and the outer diameter becomes small, the process center portion 4 stands against internal pressure. However, pressure resistance of the process end portion 5 and the heat-influenced portion 6 is low, since the strength thereof is decreased by restoration and recrystallization and the thickness thereof is not increased with the large outer diameter. Particularly, in the pressure-resistance and heat-transfer vessel having a large outer diameter, since pressure resistance is decreased in proportion to a reciprocal of the outer diameter, the thickness needs to be large. Since a phosphorus deoxidized copper tube used for a piping system connected to the pressure-resistance and heat-transfer vessel has an outer diameter of about 10 mm, a thickness of a pressure-resistance and heat-transfer vessel having an outer diameter of, for example, about 25 mm or 50 mm needs to be 2.5 times or 5 times of the thickness of the copper tube. C1220 of phosphorus deoxidized copper, which has been used for pressure-resistance and heat-transfer vessels since previous times, is easily recrystallized when a temperature thereof becomes high at the time of processing. When the temperature becomes 700° C. or higher even for a moment, crystal gains thereof are coarsened, thereby decreasing the strength.
The pressure-resistance and heat-transfer vessel is not used alone, and is used by connection with another member. The connected member is mostly a copper tube. The connection with the copper tube is performed mostly by brazing. In the brazing process, since the copper tube is excellent in heat conductivity, the copper tube is preheated widely. At the time of the connection, the process center portion 4 of the pressure-resistance and heat-transfer vessel is heated to about 800° C. or higher, which is a melting point of a general brazing material, for example, phosphorus copper lead containing 7% P. Accordingly, the process end portion 5, or the heat-influenced portion 6 as the case may be, is exposed to a high temperature of about 700° C. For this reason, a material that can stand against the heat influence at the time of the spinning process or the brazing process is necessary. Specifically, the brazing of the pressure-resistance and heat-transfer vessel, the copper tube, or the like is performed generally manually, the time of the high temperature heating is about 10 seconds and at most 20 seconds, and a material having high heat resistance is required so that the process end portion 5 and the heat-influenced portion 6 can withstand a high temperature (about 700° C.) during the time.
In the spinning process, a die or a roller is rotated at a high speed to perform drawing, and thus strength is necessary. As a material thereof, a material processed and hardened by tube rolling or drawing is used. The time of the spinning process is several seconds to several tens seconds, at most 20 seconds, and the material is greatly deformed within a short period. Accordingly, at a high temperature during the process, the material needs to be soft and satisfactorily flexible. A method for processing a drawing copper tube is represented by a spinning process of forming in a hot state. However, as described above, there is the cold-drawing processing method such as the “Hera-shibori” and the swaging of forming in a cold state. In the cold-drawing process, a long time is required since it is a cold-forming process as compared with the spinning process, but is advantageous in costs such as reduction of used materials since the thickness of the unprocessed tube portion 2 and the thickness of the drawing tube portion 3 are substantially equal to each other. However, the drawing-processed copper tube formed in a cold state has low productivity, and there is a problem in pressure resistance since the thickness of the process center portion 4 or the process end portion 5 is small. In addition, since the thickness is small, the temperature of the drawing-processed portion 8 at the time of the brazing increases as compared with the spinning process. For this reason, the drawing copper tube formed in a cold state needs to withstand increase in temperature at the time of connecting with another copper tube by the brazing, as compared with the drawing tube produced by the spinning process.
Recently, CO2 or HFC-based Freon tends to be used as a heat medium gas for a heat exchanger such as a boiler and an air-conditioner to prevent the global warming and the destruction of the ozone layer, instead of the conventionally used HCFC-based Freon. When a natural refrigerant such as HFC-based Freon and particularly CO2 is used as a heat medium, a condensation pressure needs to be increased as compared with the case of using the HCFC-based Freon gas. To withstand condensation pressure, it is necessary to further increase the thickness of the pressure-resistance and heat-transfer vessel.
When the thickness of the pressure-resistance and the heat-transfer vessel increases and thus the weight thereof increases, the cost also increases. For structural reasons and to prevent vibration, a member for fixing the pressure-resistance and heat-transfer vessel needs to be strengthened, and thus the cost further increases. Since the amount of the drawing process for producing the pressure-resistance and heat transfer vessel is increased by the increase of the thickness, the cost further increases.
A pressure-resistance and heat-transfer vessel using an inexpensive steel tube has been known, but the vessel is poor in thermal conductivity. In addition, in the spinning process, it is difficult to the drawing process as long as the temperature does not become a high temperature at which deformation resistance of a material decreases. Accordingly, it is necessary to perform sufficient preheating with a burner according to the shape, and to be 900° C. or 1000° C. or higher at the time of the processing with process heat. For this reason, a tool is overloaded and thus durability of the tool decreases. Such a steel tube is formed mainly by brazing or welding a press product, but reliability is low. Considering factor of safety, the weight of the pressure-resistance and heat-transfer vessel considerably increases.
In addition, there has been known a copper alloy tube containing Sn of 0.1 to 1.0 mass %, P of 0.005 to 0.1 mass %, 0 of 0.005 mass % or less, H of 0.0002 mass % or less, and the remainder including Cu and inevitable impurities, wherein an average crystal grain diameter is 30 μm or less (see Patent Document 1).
Since the copper alloy tube shown in Patent Document 1 is easily recrystallized at a high temperature, pressure resistance of the pressure-resistance and heat-transfer vessel processed at a high temperature after a spinning process or a brazing process is not sufficient.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-268467